Modulators are devices the object whereof is to control the transmission of a wave, for example the transmission of light in the case of optical modulators. This concept therefore covers devices that can vary the intensity of the wave transmitted continuously, or discontinuously, for example on an all or nothing basis in the case of switches.
At present the aim is to design modulators with a structure such that they can be integrated into components fabricated in silicon-based materials with a view to simplification of the fabrication of the whole and reduction of the size thereof.
In this context, it has already been proposed to modify the refractive index of a material by injection or depletion of charge carriers, for example as described in the document European Patent No. EP 1 403 684. Such modification of the index can lead to spectral displacement of a resonance of the waveguide or a shifting of the phase of the wave, which is then generally made to interfere with a wave that has not been phase-shifted (Mach-Zehnder device).
One such Mach-Zehnder device is described in U.S. Patent Application No. 2003/161565, for example, according to which the group velocity is caused to vary in one branch of the device in order to obtain a phase-shift of the signal in that branch relative to the signal that is traveling in the other branch of the device.
In both cases (displacement of a resonance or interference of phase-shifted waves), the resulting wave, and in particular its intensity, varies as a function of the density of the charge carriers, which can itself be controlled by an electrical voltage.
However, because of the physical phenomena utilized, modulation is obtained in these devices only over a narrow frequency band. Moreover, Mach-Zehnder devices have a relatively complex structure because they utilize two branches that must additionally be coupled to the main guide of the electromagnetic wave.
Modulating the absorption of a material so as to modulate directly the intensity of the wave that passes through it has also been proposed. For example, such solutions have been proposed in the papers “Current induced intersubband absorption in GaAs/GaAlAs quantum wells”, A. Fenigstein et al., PPL. Phys. Lett. 66 (19), 8 May 1995, and “Electromodulation of the interband and intraband absorption of Ge/Si cells assembled islands”, M. Elkurdi et al. Physica E16 (2003) 450/454.
However, the low absorption of silicon-based materials rules out the generation of high modulation of the intensity of the transmitted wave using known techniques, which therefore make the production of an efficient and compact modulator impossible.